This invention relates to organic light emitting diodes and, more particularly, to thermally stable, nominally amorphous films for use as emitting layers or charge transport layers in organic light emitting diodes (OLED""s). Such films are prepared from dyes that are modified to result in ionic salt analogues.
Organic light emitting diodes (OLED""s) have attracted broad attention due to their potential for use in display technologies. A few prototype displays have been demonstrated but their widespread use has not occurred because of problems associated with their operational stability and durability.
The degradation of OLED devices has been attributed to chemical changes of the organic layers, such as oxidation, and also to physical changes such as crystallization. Many potentially useful emitting or charge-transporting materials form polycrystalline films when deposited by physical vapor deposition. Such films contain grain boundaries that can act as luminescence quenching sites. To avoid such a problem, deposition of amorphous (glassy state) thin films seems to be a good alternative approach for device fabrication. However, most nonpolymeric organic dye glasses are thermally unstable and crystallize when subjected to driving voltages in a device, causing device failure.
In traditional OLED devices, the emitting organic dye molecules (D) are generally neutral molecules which interact through weak van der Waals interactions. Because of the weak nature of these interactions, heating of the initially amorphous films (for example joule heating when the device is driven, or even heating during deposition) easily causes rearrangement of the molecules, leading to crystallization and grain growth.
The film crystallization problem has been identified by many researchers in the OLED device field. Efforts to synthesize amorphous dyes have been mainly focused on dissolving them in thermally stable amorphous polymers. A second approach is aimed at synthesizing large, symmetric, rigid and dense molecules that will be more heat-resistive (see Naito et al., J. Physical Chemistry, Vol 101, p. 2350 (1997)). An alternative approach is based on the introduction of spiro centers into defined low molecular weight structures (see Salbeck et al., Synthetic Metals, Vol. 91, p. 209 (1997)).
The invention modifies dyes so as to form ionic salt derivatives. Such derivatives are used as emitting or charge transport layers in organic light emitting diodes (OLED""s). The modification involves attachment of ionizable substituents during initial synthesis, followed by formation of an ionic salt. An improvement of the OLED""s durability and performance results from use of such ionic dye salts, since the evaporated films are nominally amorphous, thermally stable and resist further crystallization. Preliminary devices built using a test compound exhibit superior efficiency.